Method of making an encapsulated impedance element



Nov. 30, 1965 GRIEST 3,220,097

METHOD OF MAKING AN ENCAPSULATED IMPEDANCE ELEMENT Original Filed Dec. 14, 1959 WIIIM Wild 000001 010' IN VEN TOR. Edward M. Griesf UM; W'

ATTORNEY United States Patent 3,220,097 METHOD OF MAKING AN ENCAPSULATED IMPEDANCE ELEMENT Edward M. Griest, Painted Post, N .Y., assignor to Corning glasks Works, Corning, N.Y., a corporation of New or Continuation of application Ser. No. 859,342, Dec. 14, 1959. This application Apr. 2, 1963, Ser. No. 270,508 7 Claims. (Cl. 29-15569) This application is a continuation of application Serial No. 859,342 filed December 14, 1959, now abandoned.

This invention relates to impedance devices and more particularly to a method of encapsulating or hermetically sealing impedance elements and the resulting structure.

Impedance devices, such as resistors, capacitors or inductors are usually encapsulated to provide the element with a thermal barrier, or to protect the element from attack by excessive moisture, excessive heat or damage by corrosion or to perform the function of electrically insulating the element from adjacent elements or, in certain applications, all functions may be served.

While I shall describe my novel process and the product resulting therefrom in terms of forming an encapsulated resistor, the preferred embodiment, it is to be understood that I do not wish to be so limited since the invention is not restricted solely to resistors.

The prior art methods of resistor encapsulation, fall into two general categories, the first of which is a potting method whereby the resistance element is coated with an appreciably thick layer of potting material. The potting material is usually in a fluid or semi-fluid state when initially applied to the resistance element, and is subsequently allowed to harden about the body of the element to provide the necessary protective coating. The other method is one where the resistance element is hermetically sealed in a container that may be either evacuated or filled with an inert atmosphere.

In either case, all known methods of scaled resistor fabrication call for the prior formation or manufacture of the complete resistor and then the subsequent step of either potting or sealing. Both methods have serious drawbacks particularly where small, precision, close tolerance, low ohmic resistance elements are required, in that the resistance element is subjected to the relatively high temperature of either the potting compound or the encapsulating sealing flame. It is this exposure to relatively high temperature that causes a radical change in resistance. In many instances, this variation may be great enough to cause the resulting resistance to exceed the allowable tolerance and hence be rejected. A high order of rejection brings about a relatively low selection rate thereby raising the unit cost of the acceptable items.

I have found that by combining the resistor fabricating steps with the sealing step, I am able to appreciably increase the selection rate and produce a resistor noted by itshigh order of accuracy and its relatively low manufacturing cost.

It is therefore an important object of my invention to provide a hermetically sealed resistor.

Another object of the present invention is to provide a hermetically sealed resistor noted by its high order of accuracy.

Still another object of the present invention is to provide a hermetically sealed resistor noted by its reproducibility and relatively high selection rate.

.. A further object of the present invention is to provide a hermetically sealed resistor that is noted by its ease of manufacture.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which,

FIG. 1 represents an exploded cross-sectional representation of one end of a resistance element indicating the components necessary to form an encapsulated resistor; and

FIG. 2 represents a cross-sectional representation of one end of my completed resistor in accordance with my invention.

Referring now to FIGS. 1 and 2 there is shown resistor body 14 having for example, an electroconductive film 16 deposited on the surface thereof. For a clear understanding of film 16, its characteristics and one example of its method of application, reference is made to US. Patents No. 2,564,706 and 2,564,707 issued in the name of I ohn M. Mochel and assigned to the same assignee as the subject application.

Adjacent the end of resistor body 14 is disc 20 to which lead 24 has been welded or affixed in any of many wellknown manner. Affixed to the end of disc 20 facing the adjacent end of resistor body 14 is a ceramic frit 18 having conductive particles of silver, for example, embedded therein. This silver-bearing frit has preferably been affixed to the end of resistor body 14 after film 16 has been placed thereon and it should be noted that a portion of frit 18 covers film 16. In both instances, the silver-bearing frits 18 have been baked on to improve the adhering qualities and in preparation for the subsequent operations.

Spaced about lead 24 is a toroidally shaped fusible end portion or bead 26, of glass or the like material, having a coefficient of thermal expansion compatible with that of lead 24. While bead 26 is herein depicted as toroidally shaped, it will be obvious to those skilled in the art that this bead may be either spherical or shaped like a washer, that is, it may be fiat. In any event, the outside diameter of bead 26 should be only slightly smaller than the inside diameter of the encapsulating sleeve 12 and should also have a coeflicient of thermal expansion compatible with the material used for sleeve 12. Sleeve 12 may be formed of glass or the like material.

In accordance with the teachings of my invention a suitable resistance element (14, 16) is cut to length to provide the required resistance. If necessary, film 16 may be appropriately spiralled to achieve higher resistances. The ends of the resistance blank are then dipped in a silver-bearing ceramic frit 18 which is then baked thereon. Frit 18 is preferably in the form of a slurry and consists of a low melting ceramic binder mixed intimately with silver particles, all of which is in suspension in an organic vehicle such as turpentine. I find that the slurry that has particular utility in this connection, utilizes a ceramic binder having a particle size that will pass through a mesh screen yet will be held on a 200 mesh screen. In a perferred form, the total composition of the slurry is about 68 grams of silver, about 10-20 grams of a fritted glass consisting of about 83% PbO, 8% ZnO, 7% B 0 and 2% SiO with both the frit and silver suspended in about 34 ml. of an organic vehicle such as turpentine.

The next step consists of placing the fusible end portion or bead 26 about lead 24 and applying sufficient heat to fuse the bead 26 to lead 24. After applying frit 18 to disc 20 and heating to insure that the frit 18 adheres to disc 20, the lead subassembly is ready for use.

The next step consists of placing the surface of frit 18 of the lead subassembly in fusible relationship with the surface of frit 18 of the resistance blank and placing sleeve 12 in a spaced relationship about the resistance element (14, 16) and bead 26. An appropriate sealing flame is then applied to the juncture of sleeve 12 and bead 26 to form the junction seal 28 (FIG. 2).

At this point it should be noted that during the sealing operation the heat generated by the flame is conducted through sleeve 12, bead 26 and lead 24- to disc 20. Disc 20 is thus raised to a temperature which is sufiiciently higher than the softening point of frit 18 to cause both surfaces of ceramic frit 18 to become fused. This is a radical departure from what is presently known in the art. The trend in the industry is to conduct heat away from the resistance elements to avoid injury thereto. Conversely, I choose to utilize the heat generated by the encapsulating step to perform the function of fusing disc 20 to the resistance element (14, 16) by the use of the intervening conductive ceramic frit 18.

By performing the fusing and sealing steps at the same time I am thus able to avoid the need to subject the resistance element to excessive temperatures during both the resistor fabricating step and again during the encapsulating step.

While I have shown and described my invention in terms of forming and sealing only one end of the resistor, it will be obvious to those skilled in the art that the same steps may be simultaneously or subsequently applied to the other end to form the completed resistor. Also, while I have described my resistance elements (14, 16) in terms of electroconductively coated resistors, it is obvious that other types of resistors, such as wire wound resistors, may be here employed equally as well.

Other embodiments of my invention that will now become apparent, reside in the substitution of either a capacitive or an inductive impedance element for the previously described resistance impedance element.

In the case of the capacitor, the foil or plate ends as well as one end of both terminal leads are coated with the conductive frit and, as in the case of the resistance blank, the fusion of the foil to the lead takes place during the sealing step.

Similarly, to encapsulate an inductive impedance, wire is Wound about a form and the wrie ends and the form ends are both coated over with the conductive frit. Thereafter, the fusion of the terminal lead to the inductor is accomplished during the sealing step and the process is identical with that previously disclosed with regard to the resistance element.

While I have described what is presently considered the preferred embodiments of my invention, it will be obvious to those skilled in the art that various other changes and modifications may be made therein without departing from the inventive concept, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What is claimed is:

1. The process for forming an encapsulated resistor comprising the steps of forming a resistor blank of a predetermined value, forming a pair of terminal leads, placing a portion of one of said leads in contact with each end respectively of said resistance blank, disposing a sleeve of fusible enopasulating material about said resistance blank and the contacting portions of said terminal leads in a spaced relationship therewith, providing each said terminal lead with an adherent fusible element disposed about said leads intermediate the ends thereof having a coefficient of thermal expansion similar to said encapsulating sleeve material, thermally sealing said resistance blank within said encapsulating sleeve only at said fusible elements, and simultaneosuly fusibly uniting each of said terminal lead contacting portions to said resistor blank ends substantially with heat conducted through said terminal leads without deleteriously affecting the electrical properties of said resistor.

2. The process of claim 1 comprising a further step of providing said resistor blank ends and said terminal lead contacting portions with a fusible conductive ceramic frit having a softening point lower than the temperature attained at the frit-coated portion of said terminal leads as a result of said heat conducted therethrough.

3. The process of claim 2 comprising a further step of applying said frit in the form of a slurry and thereafter baking said frit to insure adherence.

4. The process of claim 3 wherein said sleeve and said fusible elements are glass.

5. The process for forming an encapsulated resistor comprising the steps of forming a resistance blank, adherently coating the ends thereof with a conductive frit, forming a pair of terminal leads, adherently coating one end of each of said leads with a conductive frit, disposing an adherent fusible element intermediate the ends of each of said terminal leads, placing the frit-coated end of one said terminal lead in contact with one frit-coated end of said resistance blank, placing the frit-coated end of the other terminal lead in contact with the other frit-coated end of said resistance blank, disposing a sleeve of fusible encapsulating material about said blank in a spaced relationship therewith and about said fusible elements, said sleeve of encapsulating material having a coefficient of thermal expansion compatible with that of said fusible elements, applying heat to the juncture of said sleeve and said fusible elements to thermally seal said resistance blank within said encapsulating sleeve only at said fusible elements, and simultaneously fusing said frits with heat conducted through said terminal leads without deleteriously affecting the electrical properties of said resistor.

6. In the method for forming an encapsulated impedance device comprising the steps of forming an impedance element, forming a pair of terminal leads, and providing a sleeve of fusible encapsulating material, the improvement comprising coating each end of said impedance element and one end each of said leads with a conductive frit, disposing a fusible element intermediate the ends of each of said terminal leads, said fusible elements having a thermal expansion compatible with said sleeve, placing the frit-coated end of one said terminal lead in contact with one frit-coated end of said impedance element, placing the frit-coated end of the other terminal lead in contact with the other frit-coated end of said impedance element, disposing said sleeve about said impedance element in a spaced relationship therewith and about said fusible elements, applying heat to the juncture of said sleeve and said fusible elements to thermally seal said impedance element within said encapsulating sleeve only at said fusible elements, and simultaneously fusing said frits substantially with heat conducted through said terminal leads without deleteriously affecting the electrical properties of said impedance element, said conductive frits having a softening point lower than the temperature attained at the frit-coated end of said terminal leads as a result of said heat conducted therethrough.

7. The process for forming an encapsulated resistor comprising the steps of forming a resistance blank, coating the ends of said blank with a conductive frit in the form of a slurry comprising a mixture of about 68 grams of finely divided silver, and from about 10 to about 20 grams of fritted glass consisting by weight of about 83 percent PbO, 8 percent ZnO, 7 percent B 0 and 2 percent Si0 suspended in a sufficient amount of an organic vehicle to form a slurry, said silver and glass having a particle size of up to about 100 mesh, providing a pair of terminal leads, coating one end each of said terminal leads with said conductive frit, baking said frit to remove the organic constituents and to adhere it to said resistance blank and said leads respectively, disposing an adherent fusible element intermediate the ends of each of said terminal leads, placing the frit-coated end of one said terminal lead in contact with one frit-coated end of said resistance blank, placing the frit-coated end of the other terminal lead in contact with the other frit-coated end of said resistance blank, disposing a sleeve of fusible encapsulating material about said blank in a spaced relationship therewith and about said fusible elements, said sleeve of encapsulating material having a coeflicient of thermal expansion compatible with that of said fusible elements, applying heat to the juncture of said sleeve and said fusible elements to thermally seal said resistance blank within said encapsulating sleeve only at said fusible elements, and simultaneously fusing said frits with heat conducted through said terminal leads without deleteriously affecting the electrical properties of said resistor, said conductive frits having a softening point lower than the temperature attained at the frit-coated end of said terminal leads as a result of said heat conducted therethrough.

References Cited by the Examiner UNITED STATES PATENTS 2,174,374 9/1939 Beggs 338351 2,242,774 5/1941 Brurnley 338351 2,276,218 3/1942 Lemmens 338351 2,462,162 2/1949 Christensen et al. 338237 2,489,409 11/1949 Green et al. 29155.69 2,492,162 12/1949 Litton 29471.9 2,736,847 2/1956 Barnes 29-1555 2,791,480 5/1957 Larson 29-1555 2,883,502 4/1959 Rudner 338330 3,012,214 12/1961 Bronson et a1 29155.63 3,123,470 3/1964 Denison 6543 WHITMORE A. W-ILTZ, Primary Examiner.

JOHN H. CAMPBELL, Examiner. 

1. THE PROCESS FOR FORMING AN ENCAPSULATED RESISTOR COMPRISING THE STEPS OF FORMING A RESISTOR BLANK OF A PREDETERMINED VALUE, FORMING A PAIR OF TERMINAL LEADS PLACING A PORTION OF ONE OF SAID LEADS IN CONTACT WITH EACH END RESPECTIVELY OF SAID RESISTANCE BLANK, DISPOSING A SLEEVE OF FUSIBLE ENCAPSULATING MATERIAL ABOUT SAID RESISTANCE BLANK AND THE CONTACTING PORITONS OF SAID TERMINAL LEADS IN A SPACED RELATIONSHIP THEREWITH, PROVIDING EACH SAID TERMINAL LEAD WITH AN ADHERENT FUSIBLE ELEMENT DISPOSED ABOUT SAID LEADS INTERMEDIATE THE ENDS THEREOF HAVING A COEFFICIENT OF THERMAL EXPANSION SIMILAR TO SAID ENCAPSULATING SLEEVE MATERIAL, THERMALLY SEALING SAID RESISTANCE BLANK WITHIN SAID ENCAPSULATING SLEEVE ONLY AT SAID FUSIBLE ELEMENTS, AND SIMULTANEOUSLY FUSIBLY UNITING EACH OF SAID TERMINAL LEAD CONTACTING PORTIONS TO SAID RESISTOR BLANK ENDS SUBSTANTIALLY WITH HEAT CONDUCTED THROUGH SAID TERMINAL LEADS WITHOUT DELETERIOUSLY AFFECTING THE ELECTRICAL PROPERTIES OF SAID RESISTOR. 